An apparatus consistent with the present invention relates to a servo writer which writes a servo signal onto a magnetic tape.
In recent years development of a magnetic tape has been progressing aiming at an increase in the density of data stored on it. Some magnetic tapes for backing up a computer have a storage capacity of on the order of 100 GB, which have several hundreds of data tracks on them in their widthwise direction. The width of a track has increasingly been getting smaller, so has a gap between the neighboring data tracks, accordingly. Under the circumstances, a servo control is typically selected, so that the record/reproduction element of a magnetic head can trace the data tracks. The servo control is able to align the position of the magnetic head (widthwise position of a magnetic tape) with a servo signal that is written onto the magnetic tape in advance while the magnetic head is reading it. Japanese Published Patent Application 8-30942 discloses related arts.
The servo signal described above is recorded onto a servo band free from magnetization of the magnetic tape by applying a record current so that magnetization is carried out in one direction. As shown in FIG. 5, a conventional servo signal SS is generated by supplying a recording pulse current PC consisting of a zero current and a plus pulse current to a servo band SB free from magnetization in order to avoid saturation of a servo signal readout element (MR element).
When the recording pulse current PC falls in a zero current, no area except for a servo pattern SP is magnetized, as shown in FIG. 5. When the recording pulse current PC falls in a plus current, the servo pattern SP is magnetized in one direction by leak magnetic flux coming from a servo gap. As a result, the servo signal SS is written onto a magnetic tape MT. A record and reproduction unit, which detects a turning point of magnetization for the servo signal SS with the servo signal readout element in the form of a change in electric resistance, generates a differential wave form (voltage) as a readout signal. Accordingly, the larger is the change in electric resistance of the servo signal readout element, the larger is a peak voltage of the readout signal for the servo signal SS. This leads to an increase in an SN ratio for the readout signal.
As shown in FIG. 5, when the servo signal SS remarkably changes or a range of readout is augmented as a result of introducing a servo signal readout element having large width, a peak voltage of a readout signal RSL for the servo signal SS increases, accordingly.
The trends in developing magnetic tapes involve high density recording up to the order of some tens tetra bytes. This will dictates an increase in the number of data tracks, which inevitably requires that the width of a data track and the gap between neighboring data tracks be much narrower, as well as a decrease in the thickness of a magnetic tape. In this connection, a detectable amount of magnetism in reading the servo signal SS will decrease, and a change in an amount of magnetization of the servo signal SS detected by the servo signal readout element will decrease, accordingly.
In this way, an SN ratio of a readout signal RSS of the servo signal SS deteriorates because its peak voltage decreases as shown in FIG. 5. As a result, because it is not possible for a record and reproduction unit for a magnetic tape to accurately read out the servo signal SS, it is not possible to implement accurate positioning control for a magnetic head.
An applicant of the present invention filed a Japanese Patent Application (Japanese Unpublished Patent Application 2003-110396) disclosing a an invention, in which a servo signal is written onto a servo band while the signal is magnetized in an opposite direction as either one of longitudinal directions for magnetization of the servo band relative to a magnetic tape. In this way, the invention achieves a greater change rate and a change of magnetic field in reading the servo signal by a servo signal readout element.
Also, the applicant filed another Japanese Patent Application (Japanese Unpublished Patent Application 2003-116667). The document discloses a magnetic head complex (servo head write assembly) which has a DC erasing head and a servo signal writing head that are integrally formed. The DC erasing head applies magnetization to a servo band of a magnetic tape in either one of longitudinal directions relative to the magnetic tape. The servo signal writing head writes a servo signal onto the tape in an opposite direction as that of magnetization of the servo band.
The applicant has found that if there is widthwise misalignment between the magnetized zone and the servo signal, not only the servo signal does not work in the misaligned area, but also the DC magnetized zone will intrude a data band. When a data signal is recorded on to the DC magnetized zone, noise due to a DC component overlaps with the data signal.